The chance of you profiting by choosing a competitive niche like health or weight loss is extremely slim, espeically considering you are new to the game.
You need to think outside the box here and pick something that not everyone else is doing.
All you really need is the google keyword tool.
When you are first starting out look for keywords with 3000 to 10000 searches and no more than 50000 phrase match competition.
For example, when you type the keyword into google with quotes around it like "keyword", and it says that there are 20000 to 50000 search results for it but not more, then you are good to go.
If the keyword has only like 2k searches and has 50000 phrase match competition, it might not be ideal. There should be a good balance between searches and competition, you don't want to waste your time on things that are not worth it, because there are tons of better markets out there.
Ideal is 5000+ searches and less than 30000 phrase match competition, but this is tough to find. Also, you need to keep in mind that keywords need to be targetted.
Having a keyword like "i like flying kites" is not very targetted, a keyword like "i want to buy phones online now" is very targeted. Of course, these are two extremes.
Good luck.